Incandescent lamp including a fluorine gas atmosphere and a solid fluorinating agent



INVENTOR. Johann Schrder BY 3,311,777 INCANDESCENT LAMP INCLUDING A FLUORINE GAS ATMOSPHERE J. SCHRO'DER AND A SOLID FLUORINATING AGENT Filed March 11, 1964 March 28, 1967 AGENT United States Patent 3,311,777 TNCANDESCENT LAD/KP INCLUDING A FLUQRINE GAS ATMOSPHERE AND A SOLID FLUORDIAT- ENG AGENT .Tohann Schrfider, Aachen, Germany, assignor to North American Philips Company, Inc, New York, N.Y., a corporation of Delaware Filed Mar. 11, 1964, Ser. No. 351,164 Claims priority, application Germany, Mar. 28, 1963, N 22,953 5 Claims. (Cl. 313223) The invention relates to a gas-filled electric filament lamp which contains only fluorine or fluorine compounds as the reactive transport gas. The invention relates both to lamps with filaments consisting of metal, for example tungsten, and to lamps comprising filaments from nonmetals, carbon for example, or a high melting point compound of a metal and a meta loid, for example tantalum carbide.

During the operation of such lamps, the evaporating filament substance is transported back to the filament quantitatively and uninterruptedly. An essential advantage of the use of fluorine as reactive transport gas is that fluorine reacts at room temperature with nearly all metals and metalloids. If metal or metalloid of the filament distills towards colder parts of the lamp, it reacts with fluorine and the resulting volatile fluorine compound is again recycled back to the filament.

As a result of the great thermal stability of fluorine compounds, the recycling takes place principally towards the hottest parts of the filament, as a result of which a homogeneous geometry and temperature of the filament is adjusted and maintained during the whole life of the lamp. Fusing of the filament by evaporation is consequently rendered impossible by the cyclic process with volatile fluorides. In these circumstances, however, another process becomes decisive of the life of the filament.

The fluorine cyclic process ensures that material of the colder filament parts is transported towards the hotter part until the temperature along the whole filament is entirely homogeneous. However, the ends of the filament are cooled by the current supply members and as a result attacked and tapered until they finally fuse in the proximity of the current supply members. This occurs, in accordance with the concentration of fluorine, in a few seconds to minutes, when the colder ends of the filaments are entirely unprotected. If, however, the colder ends of the filament are centrally inserted into protecting tubes of fluorine-resistant material, the ends are protected against the direct contact with the fluorine and the attack can occur only by diffusion of the transport gases in the tubes. The tubes as well as the current supply members, supporting means and the like, are preferably manufactured from metals, for example copper, nickel, aluminium, magnesium, which are either slightly attacked by fluorine and fluorides or coat themselves with a thin, dense layer of fluorides which prevent the further attack. Nickel, for example, is resistant to fluorine to approximately 790 C. Alternatively, they may also consist of poorly melting fluorides, for example calcium fluoride or magnesium fluoride. In this manner, the attack and apering of the colder ends of the filaments may be slowed down so that useful lives are obtained. It is not possible, however, to avoid this attack and tapering, so that the filament finally fuses all the same and always at the same points.

Since the life of the lamp is consequently determined by the diffusion rate of the transport gases in the protecting tubes from and towards the colder ends of the filament, the following possibilities exist to slow down the diffusion rate. The gas pressure in the lamp may be made as large as possible by bringing inert gases, for example nitrogen or a rare gas, in the lamp in addition to the fluorine transport gases. The diflusion is further slowed down when the heavier rare gases with greater collision diameters are used. In addition, the protecting tubes can be made as narrow and as long as possible and the temperature in the tubes may be maintained as low as possible by cooling the protecting tubes.

However, narrow limits are imposed upon all these possibilities and the slowing down of the diffusion obtainable by these measures is only comparatively small. The improvement is not fundamental and can partly be obtained only by a considerable technical effort. It is a principal object of the invention to provide lamps which comprise considerably more effective means than aforesaid.

Since the diffusion rate is proportional to the concentration gradient in the protecting tube, the concentration of the transport gas in the bulb of the lamp must be kept as low as possible. The vapour pressure of tungsten, for example, above 3000 C. is in the order of magnitude of from 1O- -1() and that of carbon from 10 l0 mm. The partial pressure of the transport gas (in these examples WF or CF.,) need consequently lie only in this order of magnitude only. At such a low pressure, however, the absolute quantity of transport gas in the bulb of the lamp is very small. By gettering of fluorine 0n the component elements of the lamp which, as a result of the great reactivity of the fluorine can hardly be avoided, the store of fluorine would therefore soon be consumed and the cyclic process be discontinued.

In order to have a sutficient store of fluorine but also a concentration which is as small as possible of fluorine and fluorides in the gas chamber, according to the invention solid fluorizing agents are provided in the lamp, in which by solid fluorizing agents are to be understood to be a solid fluorine compound of a multivalent element, which compound is capable of releasing free fluorine under the conditions existing within said lamp. So in the burning lamp such fluorizing agents are easily oxidized to higher fluorides by the free fluorine originating from the dissociation, which fluorides react with volatile fluorine compounds at comparatively low temperatures. The lamps according to the invention with a filament consisting of metal, non-metal or a compound of a metal and a metalloid and a gas filling which consists partially or entirely of fluorine or volatile fluorides, are consequently characterized by the presence of a solid fluorizing agent, in the lamp which is in contact with the gas filling. For this purpose are to be considered independent of the composition of the filament and the gas in the lamp all solid fluorides which in chemistry are known as strong fluorizing agents, for example, cobalt fluoride, silver fluoride and the like. The function of these fluorizing agents as fluorine will be explained below with reference to a tungsten filament with a gas filling of fluorine and the addition of cobalt fluoride:

First cobalt fluoride is brought into the lamp, for example, by providing a rod of CoF -ceramic on the filament and evaporating it in the evacuated lamp by heating for a short time. In this manner a very thin transparent vapor-deposited layer of C01 is obtained on all the inner parts of the lamp. If in such a prepared lamp a fluorine cyclic process is maintained, for example, by filling the bulb of the lamp with 2 mm. NE, and 500 mm. Ar, the fluorine liberated during the thermal dissociation on the Warm filament is gettered on the walls of the bulb by the C01 In this case, the CoF layer is superficially fiuon'zed to C01 If tungsten of thefilament evaporates, it is fluorized on the walls of the bulb by the CoF and transported back to the filament as gaseous WF The C01 in this case is reduced to Col-' and so on. This double cyclic process is based on the following three reaction equations, in which the arrows indicate the direction of the course of the reaction.

but filament (l) W 3F; WF

i told wall of bulb (2) W GCoFs WF GCOFz cold wall of blllb\ l (3) GCOFa 3F2 SCOFg The concentration of fluorine and WF in the gas chamber is consequently determined by the evaporation rate of the tungsten filament and always is only so large as is just required for the recycling of the evaporating tungsten. However, if some fluorine is withdrawn from the cyclic process by gettering on other parts of the lamp, for example the metal parts of the protecting tube and current supply members, sufilcient fluorine is available in the form of CoF to maintain the transport.

As a result of this strong decrease of the concentration of fluorine and fluoride in the gas chamber, the concentration gradient and consequently the diffusion rate of the transport gases in the protecting tubes is reduced to such an extent, that the life of the filament is prolonged by orders of magnitude.

This does not only hold for filaments consisting of tungsten, but, naturally, also for other filament material, such as, for example, carbon or tantalum carbide, in which case the transport in the gas chamber occurs through CR; or CF +TaF Since the effective life of the lamps according to the invention is not dependent upon the evaporation rate of the materials of the filament and consequently upon the burning temperature, the burning temperature may be increased considerably. The burning temperature is now limited by the temperature at which the filament becomes too soft and thereby varies its geometry. However, in general, these temperatures lie far above the so far commonly used burning temperatures. This does not only hold for filaments consisting of tungsten, but also for other filament materials, in particular, for example, for filaments consisting of carbon. The so far commonly used burning temperature with filaments consisting of carbon lie at approximately 2000 C. In the lamps according to the invention the burning temperature, however, may be above 3000 C. As a result of this, the light efficiency and brightness is improved by a multiple. A further advantage is that the light efficiency does no longer decrease constantly by the evaporation of the filament and blackening, but remains completely constant during the whole life of the lamp.

The fluorizing agent may be brought in the lamp in a manner differing from evaporation. For example, the fluoride or a suspension thereof may be sprayed on the wall of the bulb and then dried and burned-in. Both the metals, for example Co or Ag, and the lower fluoride, for example CoF or AgF, may be brought into the lamp and the fluorizing to the higher fluoride, for example CoF AgF may occur in the lamp. However, also the higher fluoride, for example COF3, AgF may be directly brought into the lamp, as result of which the addition of gaseous fluoride becomes superfluous. However, the higher fluorides are hygroscopic and chemically very aggressive, all of which hampers the handling of these substances. Alternatively, instead of evaporating or spraying the walls of the bulb, parts consisting of fluorizing agent or material coated with fluorizing agent may be used, in which case they must have shapes which are favorably adapted to the convection circumstances in the lamp in order to guarantee a rapid exchange of fluorine. An even coating of all parts of the lamp by vapor deposition or burning in of a thin layer, however, is to be preferred, since in this manner a great surface is available for fluorine exchange and simultaneously all the parts of the lamp are additionally protected against the attack by fluorine. Naturally, all lamp parts may in addition first be coated with a thin layer which consists of substances which are resistant to fluorine for example by vapor deposition or burning in of MgF or CaF In this manner the parts of the lamp are even better protected against the attack by the fluorine.

The fluoride gas filling is independent of the solid fluorizing agent and may consist both of fluorine itself and the fluorides of the elements composing the filament, such as WF CF, and higher homologues, TaF and the like. Alternatively fluorides may be used which are converted thermally or chemically in the lamp to fluorine and inert dissociation product, for example NE, and other nitrogen fluorides. Naturally, inert gases may further be present in the lamp.

As a result of the double cyclic process, the diffusion is slowed down so strongly that also with very long burning times no noticeable loss of material of the colder ends of the filament occurs. As a result of this, on the one hand, the protecting tubes may be shorter and wider, which facilitates the technical constructions. On the other hand, however, also the ends of the filament may be deliberately kept colder, by making their crosssection greater than that of the rest of the filament. The light losses by the protecting tubes are decreased by the two measures.

In order that the invention may readily be carried into effect, a possible embodiment of the lamp according to the invention will now be described more fully, by way of example, with reference to the accompanying drawing.

The glass bulb 1 shown in the drawing is coated with a thin layer 2 of cobalt fluoride. The filament 3 consists of tungsten wire having a diameter of 0.17 mm. and the gas filling is a mixture of NF and argon, the partial pressure of the NF being 3 mm. and the pressure of the argon being 400 mm. The supporting means and current supply members 4 are manufactured from copper. The ends 5 of the filament having a diameter of 0.23 mm. are inserted in nickel tubes 6 having an inside diameter of 1 mm. The length of the protecting tubes is approximately 15 mm. In the lamp, which had an inner area of approximately 500 cm. 50 mg. of cobalt fluoride (CoF was evaporated to coat this area. The burning temperature was approximately 3000 C. and the life of the lamp approximately hours.

While I have shown and described the preferred embodiment of my invention, it will be understood that the latter may be embodied otherwise than as herein specifically illustrated or described and that in the illustrated embodiment certain changes in the details of construction and in the arrangement of parts may be made without departing from the underlying idea or principle of the invention within the scope of the appended claims.

What is claimed is:

1. An electric incandescent lamp comprising a filament of material from the group consisting of tungsten, carbon and tantalum carbide, an atmosphere in said lamp including fluorine as an agent with which the selected filament material forms a volatile compound, and a solid fluorinating agent, said atmosphere being in contact with said solid fluorinating agent.

2. An electric incandescent lamp as claimed in claim 1 wherein said solid fluorinating agent is from the group consisting of cobalt-tritiuoride and silverdifluoride.

3. An electric incandescent lamp as claimed in claim 2 wherein said solid fluorizing agent in the form of a thin film coating is located on the inside of the bulb wall and other compounds of the lamp within said bulb.

4. An electric lamp provided with a filament of a material selected from the group consisting of tungsten, carbon and tantalum carbide comprising a gas filling being at least partially constituted of a fluorine gas, a solid fiuorizing agent located on the inside of the bulb of said lamp and the other inner parts of said lamp being coated therewith, said solid fluorizing agent being in contact with said gas filling and which includes a solid fluorine compound of a multivalent element, which compound is capable of releasing free fluorine under 10 2/1962 Cooper 313-222 X 4/1966 Wollank et al. 3l3222 OTHER REFERENCES Young, Roland S.: Cobalt, N.Y., Reinhold, 1948, chapter 11, page 134.

JAMES W. LAWRENCE, Primary Examiner.

S. A. SCHNEEBERGER, Assistant Examiner. 

1. AN ELECTRIC INCANDESCENT LAMP COMPRISING A FILAMENT OF MATERIAL FROM THE GROUP CONSISTING OF TUNGSTEN, CARBON AND TANTALUM CARBIDE, AN ATMOSPHERE IN SAID LAMP INCLUDING FLUORINE AS AN AGENT WITH WHICH THE SELECTED FILAMENT MATERIAL FORMS A VOLATILE COMPOUND, AND A SOLID FLUORINATING AGENT, SAID ATMOSPHERE BEING IN CONTACT WITH SAID SOLID FLUORINATING AGENT. 